Automatic transfer of workpieces between stations at which some operation is performed on the workpieces is common in many manufacturing industries. For example, such "automation" is often utilized in the auto industry to transfer crankshafts, engine blocks, and the like between machine tools performing machining operations on the workpiece. Conveyors are widely employed to transfer such workpieces during assembly and testing thereof.
Synchronous transfer devices such as shuttles and lift and carry devices have long been used in which a series of workpieces are each simultaneously advanced between stations in step-by-step fashion.
Nonsynchronous devices have also been employed in which the workpieces are each transferred independently of the movement of the other workpieces. Examples of these include power roller and "delmatic" conveyors. Arrangements a re known in which pa 1 lets are individually controlled to be clutched and declutched to a conveyor chain to establish independent advance of each pallet and allow accumulation of pallets upstream of each workstation. See U.S. Pat. No. Re 25,886 issued on Oct. 26, 1965 for a "MANUFACTURING SYSTEM USING FREE FLOATING FIXTURE LINE", to D.A. Cargill for an example of such accumulation of chain driven pallets upstream of each work station.
Accumulation is a desirable feature, particularly in systems where various machine tools are tied together in an integrated manufacturing system. By "accumulation" is meant the ability to allow a series of work pieces to wait in readiness upstream of a given work station to provide "float", avoiding interruption of production if the supply of workpieces is temporarily discontinued.
Synchronous transfer devices require complex mechanical, electrical or pneumatic control components to provide an accumulation capability.
An advantage of nonsynchronous conveyors is their ability to provide an accumulation feature without the need for such complex controls.
However, nonsynchronous accumulating conveyors which have herefor been provided have utilized drive systems which have been custom engineered for each application and which involve interrelated drive components such that mechanical failure of any of these components tends to result in complete shutdown in the conveyor system as a whole, with the resultant loss of production of the entire manufacturing system. Such mechanical conveyor systems have also typically involved the use of complex mechanical components.
There has heretofor been disclosed systems in which electrically driven work piece supporting carriers or pallets nonsynchronously transport workpieces to work stations whereat manufacturing or other operations are performed on the workpieces.
These arrangements, while eliminating the complex interrelated mechanical conveyors, do not provide for an accumulation of work carriers at a work station and in addition rely on complex centralized electrical controls for directing the work carriers or pallets to a particular station.
U.S. Pat. No. 3,854,889 issued Dec. 17, 1974, to Lemelson for "AUTOMATIC PRODUCTION MACHINERY", and U.S. Pat. No. 3,796,327, issued Mar. 12, 1974 to Meyer et al for "MANUFACTURING SYSTEM", disclose examples of such systems.
In the prior art palletized transfer systems there has typically been required complex locating and clamping mechanisms to accurately position the workpieces in the station to achieve the degree of accuracy necessary for performing the operation or further handling of the workpieces.
There have also been previously disclosed systems in which electrically driven cars, normally stored in a holding area, transport containers of small items to various remote stations, such as in a warehouse or processing plant. There has not been disclosed the use of such electrically driven cars in an accumulating conveyor system for sequential multistation manufacturing or other operations in which workpieces are fixturized accurately on the carrier to enable operations to be performed on the workpiece while mounted on the carrier.
U.S. Pat. No. 4,015,537 issued Apr. 5, 1977 to Graef et al for "INTERIOR RAILWAY TRANSPORTATION SYSTEM", and U.S. Pat. No. 4,056,063 issued Nov. 1, 1977, for a "DISPATCHER FOR CONVEYOR SYSTEM", issued to Ritter, are examples of such disclosed systems.
Graef et al, U.S. Pat. No. 4,015,537 discloses the use of bumper switches used for discontinuing drive of a car upon encountering obstruction. Such obstruction detection is not disclosed as utilized in the normal functioning of the system and also is described as preferably being arranged for both the front and rear of each car, which would render the system impractical for application to accumulating conveyor systems, particularly for manufacturing. The touching contact required to trigger the bumper switch and stop the car does not allow for sufficient positional accuracy of the car in the stopped position for many of the applications in automated handling of workpieces.
Each of the Graef et al and Ritter patents contemplates complex electrical controls for the directing of cars to a particular station.
It is important, particularly in manufacturing plants, that the simplest design possible be employed in such conveyor systems, since increased complexity adversely affects reliability of operation of the conveyor system, particularly in integrated manufacturing systems in which a large number of conveyors are employed in the same system. Productivity of the overall system would be adversely affected by a lack of reliability in any of the individual systems incorporated .
The aforementioned U.S. Pat. No. Re 25,886 also discloses a mechanical sensing of a next adjacent pallet, and declutching from a conveyor drive chain, as does U.S. Pat. No. 4,461,216 issued on July 24, 1984 to Carney for an "OVER AND UNDER ACCUMULATING POWER AND FREE CONVEYOR SYSTEM", and U.S. Pat. No. 4,338,864 issued on July 13, 1982 to Ziegenfus for an "ASSEMBLY LINE DRIVERLESS VEHICLE".
The aforementioned U.S. patent to Carney, U.S. Pat. No. 4,461,216 also discloses vertically spaced runs of an accumulating conveyor, allowing return of pallets beneath an upper run, with pusher dogs deactivated by sensing of another pallet ahead to enable accumulation in both directions.
The nature of the drive involves complex mechanical drive components.
U.S. Pat. No. 4,552,260 issued on Nov. 12, 1985 to Teagno et al for a "WORKPIECE FEEDING APPARATUS", describes an over and under synchronous transfer of pallets, with lift mechanisms at either end of the upper and lower conveyor runs, which arrangement also involves complex mechanical drive components.